As a manufacturing method of liquid crystal display cells, the so-called liquid crystal dropping process, which has a high mass productivity, is proposed accompanying the recent years' upsizing of liquid crystal display cells (see Patent Document 1 and Patent Document 2). Specifically, the manufacturing method involves dropping a liquid crystal inside a liquid crystal sealing material formed on one substrate, and thereafter laminating the other substrate thereon to seal the liquid crystal.
However, the liquid crystal dropping process, since a liquid crystal sealing material in an uncured state first contacts with a liquid crystal, has such a problem that components of the liquid crystal sealing material dissolve in the liquid crystal on contacting and the specific resistance value of the liquid crystal decreases, thereby generating display defects in the vicinity of the seal.
As curing methods of a liquid crystal sealing material after a substrate is laminated in the liquid crystal dropping process, there are three methods of heat-curing, photocuring and combined photo- and heat-curing. The heat-curing method has such problems that a liquid crystal leaks from a liquid crystal sealing material rendered low in viscosity during curing due to the expansion of the liquid crystal by heating, and that components of the liquid crystal sealing material rendered low in viscosity dissolve in the liquid crystal. These problems are difficult to solve, so the heat-curing method is not yet in practical use.
On the other hand, liquid crystal sealing materials used in the photocuring method include two kinds of cationic polymerization type and radical polymerization type depending on the kinds of photopolymerization initiators. Since a liquid crystal sealing material of cationic polymerization type (see Patent Document 3) generates ions in photocuring, when it is used in the liquid crystal dropping process, it has such a problem that the ion components elute in the liquid crystal in the contact state and the specific resistance value of the liquid crystal is reduced. Then, since a liquid crystal sealing material of radical polymerization type (see Patent Document 4) has a large curing contraction on photocuring, it has a problem of an insufficient adhesion strength. A problematic point related to both photocuring methods of cationic polymerization type and radical polymerization type involve such a problem that metal wiring parts of an array substrate for liquid crystal display cell and black matrix parts of a color filter substrate generate light-shielded parts of a liquid crystal sealing material where light does not enter, thus causing the light-shielded parts to become uncured.
The heat-curing method and the photocuring method have such various problems, and actually the combined photo- and heat-curing method is believed to be the most practical method (see Patent Document 5). The combined photo- and heat-curing method is characterized in that a liquid crystal sealing material interposed between substrates is irradiated with light to be primarily cured, and is thereafter heated to be secondarily cured. The important characteristics required for a liquid crystal sealing material used in the combined photo- and heat-curing method involve that the liquid crystal sealing material does not contaminate a liquid crystal in every process before and after light irradiation and before and after heat-curing, and especially coutermeasures for the light-shielded parts described before, namely, countermeasures against the elution of the sealing material components from the parts of the sealing material which have not be photocured into the liquid crystal on heat-curing are required. As solving methods, the following are conceived;, (1) curing rapidly at a low-temperature before sealing material components elute out, (2) constituting components of a sealing material which hardly elute in a liquid crystal composition, and the like. Of course, the rapid curing at low-temperature accompanies worsening of the pot life during use, which is a practically big problem. Therefore, to obtain a liquid crystal sealing material which has a long pot life and exhibits a low contamination of a liquid crystal, the liquid crystal sealing material is required to be constituted of components which hardly elute in a liquid crystal composition.
However, photopolymerization initiators such as an acetophenone initiator, a benzoin initiator and a benzophenone initiator have such problems that they easily elute in a liquid crystal, and further decrease the specific resistance value of the liquid crystal when they contact with the liquid crystal.
Patent Document 6 describes a photopolymerization initiator. The described photopolymerization initiator is conceived to decrease the elution during the heating process because the initiator is captured in the cured material after photopolymerization since the initiator has both a photoreactive group (acryloyl group) and a photoinitiation site in the molecule. Considering from the structure, the initiator is supposed to be synthesized by a successive addition reaction or simultaneous addition reaction using three components of IPDI (isophorone diisocyanate), 2-hydroxy-4′-hydroxyethoxy-2-methylpropiophenone and HEA (2-hydroxyacrylate) as raw material compounds. However, it is hardly considered that the structural formula described in said patent document generates photoradicals. Besides, since the initiator is a three-component synthesized product, a problem of the contamination into liquid crystal still remains to be solved due to large amounts of by-products and residual raw material components. Further, the isolation and purification of the target compound is predicted to be costly and industrially difficult.
Patent Document 7 also describes a photopolymerization initiator. However, this initiator also is based on the similar concept and is a three-component synthesized product. Therefore, problems of the contamination into liquid crystal owing to by-products and residual raw material components and of the difficulty in the isolation and purification still remain to be solved.
As described above, there is still no satisfying photopolymerization initiator for the application to liquid crystal sealing materials of combined photo- and heat-curing type for the use in the liquid crystal dropping process in view of the liquid crystal contamination.
[Patent Document 1]: Japanese Patent Application Laying Open (KOKAI) No. 63-179323
[Patent Document 2]: Japanese Patent Application Laying Open (KOKAI) No. 10-239694
[Patent Document 3]: Japanese Patent Application Laying Open (KOKAI) No. 2001-89743
[Patent Document 4]: Japanese Patent Application Laying Open (KOKAI) No. 01-243029
[Patent Document 5]: Japanese Patent No. 2846842
[Patent Document 6]: Japanese Patent Application Laying Open (KOKAI) No. 2001-133794
[Patent Document 7]: Japanese Patent Application Laying Open (KOKAI) No. 2004-163763